Messina is a former campaign manager and chief of staff to Montana Sen. Max Baucus, who is running for re-election in 2014 and has already drawn two GOP challengers. Franken, a former “Saturday Night Live” writer and actor, is also running next year, though no major Republican opponent has yet to step forward.

Baucus, Sen. Jon Tester and Gov. Steve Bullock will also address attendees at the Lewis and Clark County Fairgrounds. Other speakers include Lee Saunders, president of the American Federation of State, County and Municipal Employees, and Nancy Keenan, the former president of NARAL Pro-Choice America and a former congressional candidate in Montana.